1. Field of the Invention
The invention relates to polysilicon thin film transistors (TFTs) and a method of fabricating the same. More particularly, the invention relates to polysilicon TFTs and methods of fabricating TFTs employing a heat retaining layer to prevent and/or decrease a reduction in a melt duration time of an amorphous silicon layer when the amorphous silicon layer is exposed to light during, e.g., a crystallization process. The heat retaining layer may be formed on the amorphous silicon layer, and may be capable of absorbing heat and/or blocking heat so as to help maintain and/or increase a melt duration time of the amorphous silicon layer during a crystallization process for forming a polysilicon layer.
2. Discussion of Related Art
In general, the mobility of electrons of a thin film transistor (TFT) including a polysilicon layer is larger than the mobility of electrons of a TFT including an amorphous silicon layer. Thus, high precision and high integration can be more easily obtained with TFTs including a polysilicon layer rather than an amorphous silicon layer. To form the polysilicon layer, intrinsic amorphous silicon may be deposited on an insulating substrate to a thickness of 40 nm to 200 nm by a predetermined method, i.e., a plasma chemical vapor deposition or a low pressure CVD (LPCVD) method, and then crystallized to form the polysilicon layer.
The crystallization method may include a laser annealing method, a solid phase crystallization (SPC) method, a metal induced crystallization (MIC) method, and a metal induced lateral crystallization (MILC) method. Among them, the laser annealing method is widely studied as a method of forming a polysilicon layer. In accordance with the laser annealing method, laser energy is supplied to a substrate on which an amorphous silicon layer is deposited to melt the amorphous silicon layer, and then, the amorphous silicon layer is cooled to form a polysilicon layer.
A conventional polysilicon TFT will be described in detail with reference to FIG. 1, which illustrates a cross-sectional view of a conventional polysilicon TFT.
As illustrated in FIG. 1, the conventional polysilicon TFT may include a substrate 10, a buffer layer 11 formed on the substrate 10, and a semiconductor layer formed on the buffer layer 11. The semiconductor layer may include a polysilicon layer 12 having protrusions 14 formed at grain boundaries.
A gate insulating layer 13 may be formed on the semiconductor layer, and a gate electrode 16 may be formed on the gate insulating layer 13. An interlayer insulating layer 17 may be formed on the gate electrode 16. The interlayer insulating layer 17 may be etched so that source and drain electrodes 18a and 18b formed on the interlayer insulating layer 17 and the semiconductor layer may be electrically connected to each other through contact holes that expose a region of the semiconductor layer.
A process of crystallizing the semiconductor layer of the conventional polysilicon TFT will be described. First, the buffer layer 11 is formed on the substrate 10 and the amorphous silicon layer is formed on the buffer layer 11. The substrate 10 may be formed of glass or plastic.
Next, laser light may be radiated onto the substrate 10, and the buffer layer 11 and the amorphous silicon layer may be laminated by the excimer laser annealing method. The amorphous silicon layer being irradiated with laser light may be crystallized to form the polysilicon layer 12. During the crystallization processes, in a state where laser light is instantaneously absorbed by the amorphous silicon layer and the amorphous silicon layer is melted to an almost liquid state, crystal growth may be performed from a crystal seed that is not melted. Grain boundaries may be formed where a crystal meets adjacent crystals, and the amorphous silicon layer may be phase transformed to the polysilicon layer 12. When silicon is phase transformed from liquid to solid, the density of silicon is reduced and the volume thereof increases. Therefore, the grain boundaries where the crystals meet generally protrude in the form of a peak or mountain, thereby forming protrusions 14. The protrusions 14 may have a height of about 40 nm to about 200 nm, which may be almost equal to a height of the amorphous silicon layer.
The protrusions 14 deteriorate the interface characteristics of the channel of the TFT, and may further affect the characteristics and degree of dispersion of a device employing such a TFT.
As described above, in the conventional art, the amorphous silicon layer is formed on the glass substrate and is crystallized using laser light to form the polysilicon layer of a TFT. However, with increasing interest in flexible displays, technology for manufacturing TFTs using a conductive substrate is being researched.
When a TFT is fabricated using a conductive substrate, a thermal conductivity of the conductive substrate will generally be larger than that of a glass substrate. For example, stainless steel may be employed as a conductive substrate. The thermal conductivity of stainless steel is about 16.3 W/mK, which is about 10 times larger than the thermal conductivity of about 1.38 W/mK for a glass substrate. Therefore, when a semiconductor layer on a conductive substrate is exposed to laser light to crystallize the semiconductor layer, heat loss may increase and a melt duration time of the amorphous silicon layer may be significantly shorter than a melt duration time of an amorphous silicon layer formed on a glass substrate. As a result of the shorter melt duration time, crystal properties of the polysilicon layer may deteriorate and it may be difficult to fabricate a TFT having a mobility of about 100 cm2/Vsec or more.